1. Field of the Invention
The present invention relates to a press pad for forming press, more particularly to a press pad for a high temperature/high pressure forming press used for the manufacture of printed circuit boards, laminated sheets, facing sheets, facing veneers, synthetic resin laminates and the like.
2. Description of the Prior Art
For the manufacture of phenolic resin or epoxy resin laminated sheets it has been the usual practice to laminate the required number of the so-called prepregs in B-stage prepared by impregnating base webs such as of paper or glass cloth with a resin and pre-setting the same, put the resulting laminate, faced if necessary with a decorative paper, steel foil or the like, between a pair of mirror plates, set this between the platens of a press with press pads thereon, and heat the laminate at 150.about.190.degree. C. under a pressure of 40.about.150 kg/cm.sup.2 so as to refluidize and subsequently set the resin.
As press pads intended for such purposes have been commonly used those made of such simple materials as non-woven fabric, paper, woven fabric and glass wool or composite materials thereof are disclosed in Japanese Utility Model Publication Nos. Sho-47-13492, Sho-54-25420 and Japanese patent publication No. Sho-53-9269.
Such press pads are, however, known to be liable to relatively early remarkable and/or irregular decrease in thickness due to loss of elastic resilience or strain recovery as they are used repeatedly under high temperature and high pressure conditions. Such a decrease in thickness is bound to result in not only early deterioration of the pad's cushioning property but also variation of its rate of heat transfer, uneven distribution of heat therein and further uneven distribution of pressure applied therethrough to possibly give cause for such defects as thin spots and voids to eventually make the pad unusable. Thus, press pads of prior art had an essential shortcoming of being relatively short in usable life.
In order to eliminate such shortcoming of the quoted prior art, there have been made such proposals as to impregnate a press pad or cushion layer consisting of a fibrous material with a synthetic resin or rubber and subsequently setting the same for fixing the interlacing points of fibers so as to improve its elastic resilience as disclosed in Japanese Utility Model Publication No. Sho-52-3415, Japanese Utility Model Public Disclosure No. Sho-55-154136 etc., or to put a rigid sheet between layers of needle-punched nonwoven web with a release layer formed on the outer surface thereof and adhesive-bonding them together to provide a press pad for forming press as disclosed in Japanese Utility Model Publication No. Sho-54-1507.
With the former, however, there is noted a drawback of the good cushioning property of the pad's surface essential in particular for a cushion material as such being adversely affected and, moreover, the desired quality of fitting closely with the mirror plates being deteriorated thereby, while, with the latter, there is a problem about the durability of the proposed, allegedly improved press pad as when it is subjected repeatedly to a high pressure, the unaided rigid sheet is inevitably deformed in the direction of the pad's thickness if gradually to eventually be so-to-speak buckled, although deformation in the said direction of the pad as a whole is somewhat prevented indeed for the horizontal elongation of the nonwoven web layers is restricted by the said rigid sheet.